Using a derived table of signal strength data to locate and track a user in a wireless network

ABSTRACT

A method for locating a user in a wireless network is disclosed. A mobile computer seeking to determine its location within a building detects the signal strength of one or more wireless base stations placed at known locations throughout the building. The mobile computer uses this measured signal strength to determine its location via a signal-strength-to-location table look-up. A table of known locations within the building and the base station signal strength at those locations is searched to find the most similar stored signal strength to the signal strength detected. The location corresponding to the most similar stored signal strength is determined to be the current location of the mobile computer. Alternatively, a number of signal strengths from the table can be used and the corresponding locations can be spatially averaged to determine the location of the mobile computer. The table can be derived empirically, by placing a mobile computer at the known locations and detecting the signal strength of the wireless base stations at those locations, or the table can be derived mathematically by taking into account a reference signal strength, the distance between the reference point and the known location, and the number of walls between the reference point and the known location. As an alternative, the base stations can detect the signal strength of the mobile computer. In such a case, the table would relate a known position of the mobile computer to the signal strength of the mobile computer at that location as detected by the one or more base stations.

TECHNICAL FIELD

This invention relates generally to determining the location of anobject and tracking the object and, more particularly, relates tolocating and tracking a user of a wireless network.

BACKGROUND OF THE INVENTION

Advances in the Global Positioning System (GPS) have providednon-military users an inexpensive and portable location and trackingdevice. Currently the GPS system is used to provide directions todrivers through an in-vehicle system, provide location and trackinginformation for marine navigation, and allow shipping companies tolocate and track individual shipments. However, the GPS system isseverely limited in an indoor environment.

The GPS system relies on the timing of signals from GPS satellitesreceived by individual GPS units on the ground. Thus, an unobstructedview to the satellites is necessary to receive the signal. In an indoorenvironment, such an unobstructed view is, in general, not possible toobtain. Furthermore, the principal objective in developing GPS was tooffer the United States military accurate estimates of position,velocity and time. Civil users of GPS were to be provided only“reasonable” accuracy consistent with national security concerns. As aresult, satellite signals are purposefully degraded under a governmentpolicy called Selective Availability and consequently the resolutionprovided by the system is no more than 100 meter for civilian users.This coarse resolution is inadequate for many applications and compoundsthe problem of the ineffectiveness of GPS indoors.

Because of these limitations, other technologies have been developed tolocate and track users or objects in an in-building environment. Onesuch system uses tags placed on the items that are to be tracked. Thetags can be either active or passive. An active tag contains powercircuitry, which can communicate with base stations. A passive tagcontains no internal power, rather it is charged either inductively orelectromagnetically as it passes within the range of a base station.Using this derived power, the passive tag communicates with the station.The base stations are physically linked together through a wired orwireless network. Each tag transmits a code uniquely identifying itselfThus, the location of the tag is determined to be in the vicinity of thebase station with which the tag last communicated.

Such tag-based tracking and location systems, while being useful in anin-building environment, require a significant installation ofspecialized base stations. A tag-based system can only determine thelocation of the tags as being “near” a particular base station,consequently, to achieve a sufficiently high resolution a large numberof base stations must be installed. Obtrusive tags have to be placed onevery item that is to be tracked or located, and in the case ofinfra-red tags, the system operates only when there is a “line-of-sight”between the tag and a base station. For all these reasonslocation-determination technology based on tags has had very limitedsuccess.

SUMMARY OF THE INVENTION

Therefore, the present invention is generally directed to a system forlocating and tracking a user in a building without a specializedinfrastructure and with the ability to track without a line-of-sightbetween the user and a base station.

The present invention is also generally directed to a system forlocating and tracking a user in a building using the existingRadio-Frequency (RF) Wireless Local Area Network (WLAN) infrastructure.

A Wireless Local Area Network (WLAN) consists of base stations connectedto a wired network, and mobile devices which are “connected” to the WLANthrough wireless communication with the base stations. The presentinvention uses the signal sensing ability of both the base station andthe mobile device to determine the location of the mobile device, andthus the location of the user of the mobile device. The strength of thereceived signal from several base stations is measured by the mobiledevice. The mobile device then compares the signal strength from each ofthe base stations to a pre-computed table containing the base stations'signal strength at various known locations of the mobile device. Fromthis comparison, the mobile device determines its location.Alternatively, the signal strength from the mobile device can bemeasured at a number of base stations. This signal strength is thencompared by a central computer to a pre-computed table containing themobile computer's signal strength at the base stations for various knownlocations of the mobile computer. From this table, the central computerdetermines the location of the mobile computer.

Additional features and advantages of the invention will be madeapparent from the following detailed description of illustrativeembodiments which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

While the appended claims set forth the features of the presentinvention with particularity, the invention, together with its objectsand advantages, may be best understood from the following detaileddescription taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a block diagram generally illustrating an exemplary computersystem on which the present invention resides;

FIG. 2 is a diagram generally illustrating a wireless network accordingto the present invention;

FIG. 3 is a diagram generally illustrating a wireless network on onefloor of an office building;

FIG. 4 is a diagram generally illustrating the locations of empiricaldeterminations of signal strength according to the present invention;

FIG. 5 is a diagram generally illustrating the operation of aline-clipping algorithm; and

FIG. 6 is a flow chart generally illustrating the operation of amathematical derivation of a signal strength table according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings, wherein like reference numerals refer to likeelements, the invention is illustrated as being implemented in asuitable computing environment. Although not required, the inventionwill be described in the general context of computer-executableinstructions, such as program modules, being executed by a personalcomputer. Generally, program modules include routines, programs,objects, components, data structures, etc. that perform particular tasksor implement particular abstract data types. Moreover, those skilled inthe art will appreciate that the invention may be practiced with othercomputer system configurations, including hand-held devices,multi-processor systems, microprocessor based or programmable consumerelectronics, network PCs, minicomputers, mainframe computers, and thelike. The invention may also be practiced in distributed computingenvironments where tasks are performed by remote processing devices thatare linked through a communications network. In a distributed computingenvironment, program modules may be located in both local and remotememory storage devices.

With reference to FIG. 1, an exemplary system for implementing theinvention includes a general purpose computing device in the form of aconventional mobile personal computer 20, including a processing unit21, a system memory 22, and a system bus 23 that couples various systemcomponents including the system memory to the processing unit 21. Thesystem bus 23 may be any of several types of bus structures including amemory bus or memory controller, a peripheral bus, and a local bus usingany of a variety of bus architectures. The system memory includes readonly memory (ROM) 24 and random access memory (RAM) 25. A basicinput/output system (BIOS) 26, containing the basic routines that helpto transfer information between elements within the mobile personalcomputer 20, such as during start-up, is stored in ROM 24. The mobilepersonal computer 20 further includes a hard disk drive 27 for readingfrom and writing to a hard disk 60, a magnetic disk drive 28 for readingfrom or writing to a removable magnetic disk 29, and an optical diskdrive 30 for reading from or writing to a removable optical disk 31 suchas a CD ROM or other optical media.

The hard disk drive 27, magnetic disk drive 28, and optical disk drive30 are connected to the system bus 23 by a hard disk drive interface 32,a magnetic disk drive interface 33, and an optical disk drive interface34, respectively. The drives and their associated computer-readablemedia provide nonvolatile storage of computer readable instructions,data structures, program modules and other data for the mobile personalcomputer 20. Although the exemplary environment described herein employsa hard disk 60, a removable magnetic disk 29, and a removable opticaldisk 31, it will be appreciated by those skilled in the art that othertypes of computer readable media which can store data that is accessibleby a computer, such as magnetic cassettes, flash memory cards, digitalvideo disks, Bernoulli cartridges, random access memories, read onlymemories, and the like may also be used in the exemplary operatingenvironment.

A number of program modules may be stored on the hard disk 60, magneticdisk 29, optical disk 31, ROM 24 or RAM 25, including an operatingsystem 35, one or more application programs 36, other program modules37, and program data 38. A user may enter commands and information intothe mobile personal computer 20 through input devices such as a keyboard40 and a pointing device 42. Other input devices (not shown) may includea microphone, joystick, game pad, satellite dish, scanner, or the like.These and other input devices are often connected to the processing unit21 through a serial port interface 46 that is coupled to the system bus,but may be connected by other interfaces, such as a parallel port, gameport or a universal serial bus (USB). A monitor 47 or other type ofdisplay device is also connected to the system bus 23 via an interface,such as a video adapter 48. In addition to the monitor, personalcomputers typically include other peripheral output devices, not shown,such as speakers and printers.

The mobile personal computer 20 may operate in a networked environmentusing logical connections to one or more remote computers, such as aremote computer 49. The remote computer 49 may be another personalcomputer, a server, a router, a network PC, a peer device or othercommon network node, and typically includes many or all of the elementsdescribed above relative to the mobile personal computer 20, althoughonly a memory storage device 50 has been illustrated in FIG. 1. Thelogical connections depicted in FIG. 1 include a Wireless Local AreaNetwork (WLAN) 51 and a wide area network (WAN) 52. Such networkingenvironments are commonplace in offices, enterprise-wide computernetworks, intranets and the Internet.

When used in a WLAN networking environment, the mobile personal computer20 is connected to the local network 51 through a wireless networkinterface or adapter 53. The wireless interface 53 transmits wirelesspackets to a base station 61. The base station 61 can then retransmitthe packets, either through a wired or wireless network to the remotecomputer 49. When used in a WAN networking environment, the personalcomputer 20 typically includes a modem 54 or other means forestablishing communications over the WAN 52. The modem 54, which may beinternal or external, is connected to the system bus 23 via the serialport interface 46. In a networked environment, program modules depictedrelative to the mobile personal computer 20, or portions thereof, may bestored in the remote memory storage device. It will be appreciated thatthe network connections shown are exemplary and other means ofestablishing a communications link between the computers may be used.

In the description that follows, the invention will be described withreference to acts and symbolic representations of operations that areperformed by one or more computers, unless indicated otherwise. As such,it will be understood that such acts and operations, which are at timesreferred to as being computer-executed, include the manipulation by theprocessing unit of the computer of electrical signals representing datain a structured form. This manipulation transforms the data or maintainsit at locations in the memory system of the computer, which reconfiguresor otherwise alters the operation of the computer in a manner wellunderstood by those skilled in the art. The data structures where datais maintained are physical locations of the memory that have particularproperties defined by the format of the data. However, while theinvention is being described in the foregoing context, it is not meantto be limiting as those of skill in the art will appreciate that variousof the acts and operation described hereinafter may also be implementedin hardware.

In accordance with the invention, an exemplary WLAN is shown in FIG. 2.The base stations 72, 74, and 76 are the same as base station 61 in FIG.1, however, for clarity, each base station has been separately numberedso that each can be referred to individually. Similarly, mobile personalcomputers 78, 80, and 82 are all of the same type as mobile personalcomputer 20 described above. With reference to FIG. 2, the mobilecomputer 78 can communicate, via wireless communication, with eitherbase station 72 or base station 74 as shown. In a known manner, themobile computer 78 can select the base station which provides thehighest signal strength, measured by the signal-to-noise ratio (SNR).The base stations 72, 74, and 76 can be connected by connection 70,which can be either a wired or wireless network. Therefore, to link twocomputers together, the WLAN passes messages through the base stations.For example, to communicate with mobile computer 80, mobile computer 78can contact either base station 72 or base station 74. These basestations will then relay the message, through connection 70, to basestation 76, which is the base station with which mobile computer 80 maycurrently be communicating. Base station 76 will then transmit themessage to mobile computer 80, completing the wireless connectionbetween mobile computers 78 and 80.

Because the mobile computers 78, 80, and 82 can be transported, the WLANdefines a mechanism by which communication between the mobile computersand the WLAN is transferred from one base station to another. The mobilecomputers 78, 80, and 82 monitor the signal strength of the basestations 72, 74, and 76. In some embodiments the mobile computerscontinuously monitor the signal strength, and in others, the mobilecomputers only monitor the signal strength when the SNR of the basestation with which the mobile computer is currently communicating fallsbelow an acceptable level. Similarly, to detect and connect to mobilecomputers as they are transferred between base stations, the basestations 72, 74, and 76 monitor the signal strength from the mobilecomputers 78, 80, and 82. A more detailed description of how the mobilecomputers are passed between base stations can be found in U.S. Pat. No.5,717,688 entitled WIRELESS LOCAL AREA NETWORK WITH ROAMING INDICATINGMULTIPLE COMMUNICATION RANGES by Belanger et al., the teachings of whichare incorporated herein by reference in their entirety.

The present invention uses this monitoring of signal strength by boththe base stations and the mobile computers, to locate and track a mobilecomputer and its user. An exemplary building layout is shown in FIG. 3.The building includes hallway 90; offices 92, 93, 94, 95, 96, 97, and98; and conference rooms 100 and 102. Base stations 72, 74, and 76 havebeen placed at various locations in the building. The mobile personalcomputers 78, 80, and 82 are also in the building, although they canmove freely throughout the building.

The mobile personal computer 78 can monitor the strength of the signalfrom base stations 72, 74, and 76. As is known by those of skill in theart, the signals of a WLAN are attenuated as they propagate and as theypass through walls. Therefore, as monitored by mobile computer 78, thesignal from base station 72 is stronger than the signals from basestations 74 and 76. This is because the signals from base stations 72and 74 must travel a greater distance and must pass through more walls.

The signal strength from base stations 72, 74, and 76 will vary as themobile computer 78 is moved around the building. For example, if theuser of mobile computer 78 moved the computer into conference room 100,the signal strength of base station 74 as detected by mobile computer 78would increase, as the distance between base station and mobile computerdecreased, and as there would no longer be any walls between them.Similarly, the signal from base station 76, as detected by mobilecomputer 78, would increase in strength due to the decreased distanceand decreased number of intervening walls. However, the signal from basestations 72 would decrease, since mobile computer 78 would move furtheraway. Therefore, it is possible to create a table listing knownlocations in the building, and the corresponding signal strengths fromeach of the base stations as received at those locations.

Turning to FIG. 4, a method for creating the location versus signalstrength table is shown. In FIG. 4, diamonds with letters in themindicate locations where empirical measurements of the signal strengthof the base stations 72, 74, and 76 have been taken. These measurementscan then be compiled into a table, such as Table 1, shown below. Thus,location B has a higher SNR for base station 72 than location A because,while both are approximately equidistant from base station 72, locationA is separated from the base station by the wall of office 92. Theobservation that distance and intervening walls decrease the signalstrength, and thus decrease the SNR, is true for the rest of the entriesin the table as well. TABLE 1 SNR from SNR from SNR from Location Base72 Base 74 Base 76 A 40 dB 20 dB 20 dB B 45 dB 30 dB 30 dB C 35 dB 25 dB25 dB D 30 dB 30 dB 30 dB E 40 dB 35 dB 35 dB F 25 dB 40 dB 30 dB G 25dB 45 dB 35 dB H 20 dB 40 dB 40 dB I 25 dB 35 dB 40 dB J 30 dB 30 dB 35dB K 35 dB 25 dB 30 dB L 40 dB 20 dB 40 dB M 25 dB 30 dB 45 dB

The strength of the radio frequency signals measured by the base station76 or the mobile computer 20 can vary as a function of the orientationof the mobile computer 20 when performing the measurements. Moreparticularly, the orientation of the computer is related to the positionof the user, and it is the user's body that can create a significantdifference in the detected signal strength. It is therefore necessarythat the table used in determining the location of the mobile computertake this effect into account and be able to determine the locationregardless of the orientation of the user with respect to the mobilecomputer. One method for taking this effect into account is to considermultiple orientations of the user's body to minimize the effects ofsignal attenuation due to the user's body.

The present invention also contemplates multiple measurements at eachlocation to remove the effect of the random variables, such as aircurrents and radio interference, that can affect signal quality. Becausethese events are often short lived, multiple samples taken at onelocation over a period of time will yield different results. To removethe effect of the random variables, the calculated signal strengthvalues over a period of time are averaged. This average value is thenused in a table, such as Table 1 above. In such a manner the accuracy ofthe table, and thus the location determination, is enhanced.

Once a table relating signal strength to the position of the mobilecomputer 20, such as Table 1, is created, the mobile computer 20 candetermine its location by finding the row of the table which mostclosely corresponds to the signal strengths detected by the mobilecomputer. For example, mobile computer 78 might detect a SNR of 38 dBfrom base station 72, 23 dB from base station 74, and 24 dB from basestation 76. By comparing these values to Table 1, the mobile computer 78determines that it is located at physical position C. This determinationcan be done in signal space. The signal space is an multi-dimensionalspace where the number of dimensions is equivalent to the number of basestations' signals which the mobile computer uses to determine itslocation. In Table 1 above, three base stations' signal strengths aredetermined at each physical location, so the signal space is athree-dimensional space. As will be known by those skilled in the art,unlike physical space, the signal space is not limited to threedimensions.

Each set of three measured signal strengths from the three base stationscan define a point in the signal space. In one realization of thisinvention, the Euclidean distance between the point defined by themeasured signal strengths and the points defined by the empiricallyderived signal strengths in Table 1 can be calculated. As is known bythose of skill in the art, the Euclidean distance is the square root ofthe sum, over all the dimensions, of the difference between two pointsin each dimension, squared. In mathematical terms, the Euclideandistance, d, is defined as:${d = \sqrt{\sum\limits_{i = 1}^{n}\left( {a_{i} - b_{i}} \right)^{2}}},$where a_(i) is the value for the ith coordinate of point a and b_(i) isthe value of the ith coordinate of point b, and n is a variableequivalent to the number of dimensions in the signal space. The physicallocation of the mobile computer 20 is determined to be the same as thelocation whose corresponding empirically derived signal strengths in thetable are the closest (as defined above) to the measured signalstrengths. Thus, for the measured signal strengths of 68 dB, 53 dB, and54 dB given as an example above, the location of mobile computer 78would be determined to be location C. This is because the Euclideandistance in signal space between (38,23,24), which are the measuredvalues, and (35,25,25), the stored values at point C, which weredetermined empirically during system set-up, is less than the Euclideandistance between (38,23,24) and any other point in the table. The mobilecomputer 20 therefore concludes that it is located at location C. Thelocation of a mobile computer 20 can thus be determined through sensingthe signal strength from each of a number of base stations. As is knownby those skilled in the art, the Euclidean distance is just one possibledistance metric. Other distance metrics such as sum of absolute valuedifferences, or weighted Euclidean are also possible.

Rather than merely comparing the detected values to a single row of thetable, the present invention also contemplates finding several rows ofthe table, each of which contains values similar to those observed. Sucha multiple nearest neighbor approach, spatially averages multiplelocations at which the empirically determined signal strengths from thebase stations are similar to those measured by a mobile computer seekingto determine its location. As will be known by those of skill in theart, spatial averaging involves averaging the individual coordinatevalues of the locations. Therefore, it is necessary to define aconsistent coordinate system, such as using a corner of the building asthe origin (0,0). Returning to the example above, the values detected bymobile computer 78, namely 38 dB, 23 dB, and 24 dB, respectively aresimilar to the entries for a number of rows in addition to row Cselected above. Using the multiple nearest neighbor approach, we findrows A and B are also similar to the detected values. These three“nearest neighbors”: A, B, and C, are spatially averaged to determinethe location of mobile computer 78. As will be apparent to one of skillin the art, the multiple nearest neighbor approach can be implementedwith any number of “neighbors”.

A variation of the multiple nearest neighbor approach using weighting isalso contemplated by the present invention. If one set of signalstrength values are very similar to the values detected by the mobilecomputer 20, it may be that the position corresponding to those valuesis more accurate than any other position at which an empiricalmeasurement was taken. That does not mean, however, that the multiplenearest neighbor approach cannot improve the accuracy of that positiondetermination. However, because there exists a position which appears tobe near the actual position of the mobile computer, only minor changesto that position should be caused as a result of the multiple nearestneighbor approach. In such a case, a weighted multiple nearest neighborapproach may be appropriate. A weighted multiple nearest neighborapproach multiplies the coordinates of each “neighbor” location by aweighting factor prior to averaging them. If a position appears to be aparticularly good match, then the coordinates of that position would bymultiplied by a larger weighting factor than the other positions. Insuch a manner, the weighted multiple nearest neighbor approach wouldcause less deviation from the perceived best position, yet would providea minor position adjustment which could result in even greater accuracy.

The present invention also contemplates locating a mobile device bymeasuring the signal strength from the mobile device as received byseveral base stations. As will be evident to those skilled in the art,such a method is the inverse of the method described above. Returning toFIG. 2, a computer 84 can also be connected to the base stations 72, 74,and 76, through the network 70. Computer 84 can monitor the signalstrength from a particular mobile computer 20 as received by the basestations near that mobile computer. Because it is connected to all ofthe base stations, the computer 84 can also generate the table whichrelates the position of the mobile computer 20 to the strength of thesignal from the mobile computer as received by each of the basestations. Returning to FIG. 4, as explained above, empirical data can begathered by sampling the signal strength from the mobile computer, as itis moved throughout the building. In this case, however, the tablegenerated by computer 84 as a result of the sampling will contain thesignal strength of the signal from the mobile computer as detected ateach base station, instead of the signal strength of the signal fromeach base station as detected at the mobile computer. The computer 84can collect the signal strength information as received by each basestation and create a table such as Table 2 below. TABLE 2 Location ofSNR at SNR at SNR at User Base 72 Base 74 Base 76 A 30 dB 10 dB 10 dB B35 dB 20 dB 20 dB C 25 dB 15 dB 15 dB D 20 dB 20 dB 20 dB E 30 dB 25 dB25 dB F 15 dB 30 dB 20 dB G 15 dB 35 dB 25 dB H 10 dB 30 dB 30 dB I 15dB 25 dB 30 dB J 20 dB 20 dB 25 dB K 25 dB 15 dB 20 dB L 30 dB 10 dB 30dB M 15 dB 20 dB 35 dB

As can be seen from a comparison of Table 1 above and Table 2 above, thetables are similar with respect to the relationship between the signalstrengths in any one row. Even though the signal strength measured atthe mobile computer 20 and the base station 76 are similar in value,there is no requirement that they be so. The two signals travel the samepath and encounter the same obstacles which degrade the signal. The onlydifference can be the power of the transmitting devices themselves: thebase station, since it does not need to conserve power, may betransmitting at a higher power than the wireless network interface 53 onthe mobile computer 20. For example, the Federal CommunicationsCommission (FCC) allows wireless networks to use up to 1 Watt oftransmitting power, which can be easily met by the base station. Thewireless network interface 53, however, typically transmits in the50-100 mW range so as to conserve the battery power of the mobilecomputer 20.

Therefore, as can be seen by comparing Table 1 and Table 2, the signalsreceived by the base stations in Table 2 are weaker than those receivedby the mobile computer from the base stations in Table 1. However,because the signals in both directions are equally affected by distanceand obstacles, the relationship between the signals in a row remains thesame. For example, at location A in Table 1 the signal from base station72 is stronger than the signals from base stations 74 and 76 because ofthe distance and the obstacles between location A and base stations 74and 76. This is the same reason that in Table 2, at location A, thesignal from the mobile computer at location A is stronger when detectedby base station 72 than when detected by base stations 74 and 76.Because the mobile computer does not transmit its signals with as muchpower, the SNR is lower in Table 2 than in Table 1.

As described above, multiple measurements taken at several differentorientations of the mobile computer 20 can be used to eliminate theeffect of the user's body on the signal strength when creating a tablesuch as Table 2. Additionally, as described above, taking multiplemeasurements at each location can improve the accuracy of the values inthe table, as it minimizes the effects of random interference and noise.

The multiple nearest neighbor method described in detail above can alsobe used with the data in Table 2. For example, the computer 84 candetermine that the strength of the signal from mobile computer 78, asshown in FIG. 4, is 28 dB as detected by base station 72, 13 dB asdetected by base station 74, and 14 dB as detected by base station 76.Comparing these values to Table 3, we find locations A, B, and C as thenearest “neighbors” to the values obtained. The nearest neighbors arethen spatially averaged to calculate the location of mobile computer 78.As above, the use of the multiple nearest neighbor method requires theuse of a consistent coordinate system. Also, a weighted multiple nearestneighbor approach, as described above, can be used with a table such asTable 3.

An alternative method to empirically deriving Tables 1 or 2 above,contemplated by the present invention, requires mathematicallyestimating the attenuation in the signal as due to the distance betweenthe transmitter and the receiver, and the intervening walls. As is knownby those skilled in the art, the transmissions in a wireless networkconform to general radio propagation theories. Most notably the signalstrength of the transmissions becomes weaker as the distance from thetransmitting source increases, and the signal strength of thetransmissions decreases when the signals must pass through walls.Research into the field of electromagnetic waves has yielded a number ofuseful mathematical formulas. For example, it is known that signalstrength decreases with distance. It is also known that walls attenuatesignals by a determinable factor, known as the Wall Attenuation Factor(WAF). The WAF is dependent on the thickness of the wall, and thematerials used in the construction of the wall. The WAF can bedetermined empirically by placing a receiver on one side of a wall and atransmitter on the other, and detecting the attenuation of the signalthrough the wall. One method for determining a WAF can be found in thearticle entitled “914 MHz Path Loss Prediction Models for IndoorWireless Communications in Multifloored Buildings” by Scott Y. Seideland Theorodre S. Rappaport, which appeared in IEEE Transactions OnAntennas and Propagation, Volume 40, Number 2, February 1992, theteachings of which are hereby incorporated by reference.

The signal strength at a particular location can, therefore, be definedas the signal strength as attenuated by distance and walls that thesignal had to pass through. In mathematical terms, the formula can beexpressed as follows: the signal strength, or power P, at a givendistance d is calculated as${P(d)} = {{P\left( d_{0} \right)} - {10n\quad{\log\left( \frac{d}{d_{0}} \right)}} + \left\{ {\begin{matrix}{{nW}({WAF})} & {{nW} < C} \\{C({WAF})} & {{nW} \geq C}\end{matrix}.} \right.}$The first term is the signal strength at a reference distance d₀. Thereference distance can be chosen to be a convenient distance and thesignal strength at that distance can be determined empirically. If thereference distance is chosen to be one meter from the emitter itself,then the signal strength may be available from the specifications of thedevice, eliminating the need for empirical determination. The secondterm in the equation provides the attenuation in the signal strength dueto the distance between the point at which the signal strength is soughtto be calculated, and the reference point at which the signal strengthis known. The variable n is the path loss component which indicates therate at which the signal strength decreases with distance. The secondterm is subtracted from the first term, and thus, reduces the referencesignal strength if the desired point is further from the transmitterthan the reference point. The third term in the equation quantifies theattenuation due to the number of walls the signal must pass through. Theterm nW represents the number of walls and the WAF can be determinedempirically, as described above. Thus, the product of the two yields theattenuation due to all of the walls. As explained above, the WAF is anegative number, resulting in a reduction to the calculated signalstrength. After a certain number of walls, however, the signal becomesso weak that further degradation is not mathematically significant. Thispractical limit is C walls, where C can be selected by the user tosatisfy that user's accuracy requirements. After C number of walls,therefore, the attenuation factor can simply be expressed as Cmultiplied by the WAF.

The number of walls between a signal source and the location for whichthe signal strength is to be calculated can be determined by automatedmeans. For example, in one realization of this invention, the buildingis divided into non-overlapping but connected rectangular regions, asdefined by the walls inside the building. A straight line between thebase station 76 and the location of the mobile personal computer 80,indicating the path the signal would take when no walls are present areconsidered. Cohen-Sutherland Line Clipping Algorithm can then be used todetermine the number of walls between the source and receiver. Lineclipping is the deletion of a part of a line segment which lies outsidea clip area. Once the line clipping is performed, the remaining linesegment lies completely within a clip area. The algorithm divides theregion outside of the clipping area into areas with “outcodes”. Eachdigit of the binary outcode represents whether the particular regionlies above, below, or to the left or right of a clipping area. As isshown in FIG. 5, a four binary digit outcode can be used. The firstdigit indicates whether the region is above the clipping region 104. Forexample, regions 118, 120, and 106 are all above region 104, asindicated by the dashed lines, and those regions all have a binary 1 asthe value of the first digit. The second digit indicates whether theregion is below the clipping region 104. Thus, regions 114, 112, and 110all have a binary 1 as the value of the second digit since they arebelow the clipping region 104. Regions 108 and 116 are adjacent to theclipping region and therefore both the first and second binary digitsare 0, since adjacent regions are neither above nor below the clippingregion. Similarly, the third binary digit indicates whether the regionis to the right of the clipping region 104, and the fourth binary digitindicates whether the region is to the left of the clipping region. Theclipping region 104 itself has an outcode of 0000 because, bydefinition, it is nether above, nor below, and neither to the left norto the right of itself.

Once this subdivision occurs, all lines through the clipping region,such as lines 122 and 124 shown in FIG. 5, are divided by the boundariesof the regions described above, as shown at points 126 and 128 for line122, and points 130 and 132 for line 124.

A key feature of the outcodes is that the binary digits which have avalue of 1 correspond to boundaries between the regions, which arecrossed. For example, line 122 has two endpoints: one in region 0000 andanother in region 1001. The significance of outcode 1001 is that theline must intersect the top and left boundaries because the first digitindicates it is above the clipping region and the last digit indicatesit is to the left of the clipping region, as described above. Whenever apoint on the line changes its outcode value, by definition the line hascrossed a boundary. It is possible that this line intersects a number ofrectangular regions. For example, the office layout from FIGS. 3 and 4can be divided into regions with outcodes as shown in FIG. 5. TheCohen-Sutherland Line Clipping Algorithm is described further in Chapter3.11 of Computer Graphics Principles and Practice, Second Edition byFoley et al., published in 1990, the teachings of which are incorporatedherein by reference.

Turning to FIG. 6, at step 140, the floor schematic is entered into acomputer as an input to the Cohen-Sutherland Line Clipping Algorithm.Similarly, at step 142 the locations of the base stations are entered,and at step 144 the locations at which the signal strength is to becalculated are entered. The Cohen-Sutherland Line Clipping Algorithm atstep 146 will then determine, as described above, the number of wallsbetween each of the base stations and the locations at which the signalstrength is to be calculated. This number is then used in the signalstrength equation, given above, at step 148.

In addition to the number of walls, the WAF, empirically derived asdescribed above, is entered at step 150. At step 152, the limitingfactor C is selected by the user. The variable n, the rate at whichsignal strength decreases with distance, is provided at step 154. Thereference distance and the signal strength at that reference distance,as described above, are provided in steps 158 and 156 respectively. Thesignal strength equation given above can then, at step 148, calculatethe signal strength at each of the locations entered in step 144. Theoutput of the signal strength equation is entered into a table, such asTable 1 or Table 2, above, at step 160.

The mathematical estimation can be used to derive both a tablecontaining the values of the signal strength from various base stationsas detected by a mobile computer 20 and the values of the signalstrength from a mobile computer 20 as detected by various base stations.As would be known by those of skill in the art, the variables in thesignal strength equation would be different depending on which methodwas used. For example, to calculate the signal strength from a basestation, as detected by a mobile computer at a particular location, thereference distance should be set close to the base station, or even atone meter from the base station, as described above, and the term nwould be the rate at which the signal strength of the base stationdecreased with distance. Conversely, to calculate the signal strengthfrom a mobile computer 20 at a particular location, as detected by abase station, the reference distance should be set close to the mobilecomputer, such as one meter away, and the term n would be the rate atwhich the signal strength of the mobile computer 20 decreased withdistance.

The present invention also contemplates providing for environmentalfactors by environmentally profiling the system. The base stations areall located at known locations, as shown in FIGS. 3 and 4. One basestation could send out a signal to be received by the other basestations, in the same way that the base stations would receive a signalfrom a mobile computer. For example, with reference to FIG. 3, basestation 72 could emit a signal to be detected by base stations 74 and76. Using a table generated by the signal strength equation given above,the computer 84 could receive the signal of base station 72, as detectedby base stations 74 and 76, and determine the location of base station72. This would be done in the same manner that computer 84 woulddetermine the location of a mobile computer 20 from its signal strengthas detected at the base stations, as described above.

The computer 84 can then compare the calculated location to the knownlocation of base station 72. If there is a difference between the two,the inputs to the variables in the signal strength equation, given abovefor estimating the signal strength at a particular location, can bevaried. For example, the n value, or the WAF, could be changed, and anew table recalculated. This new table would then be used by computer 84to calculate again the position of base station 72. The computer 84could then again compare the new calculated location of base station 72to the known location and determine the difference. The above iterationcan be performed multiple times. The computer 84 could determine whichvalues of the variables in the signal strength equation yielded the mostaccurate results, and then use those values to generate the table, whichwould, in turn, be used to calculate the position of the mobilecomputers. The values of the variables could also be passed to themobile computer 20 along with a map of the building, so that it cangenerate a table within itself, and determine its location based on thestrength of the signal from various base stations, as described above.As would be known by those of skill in the art, the variables in thesignal strength equation could be varied beforehand, generating numeroustables. The above method could then be used to select the most accuratetable of the group generated. In either case, the environmentallyprofiled tables could result in more accurate calculations. Becausefactors which affect signal strength, such as the number of peoplepresent, can vary, the system can be programmed to profile and tuneitself, as described above, multiple times.

An alternative environmental profiling mechanism can be implemented byempirically deriving more than once the table relating the position ofthe mobile computer to the signal strength. As described above, a tablerelating the position of the mobile computer to the signal strength,such as Tables 1 or 3, can be derived empirically. Such empiricalderivations can be performed numerous times, under differentenvironmental conditions, and at different times of the day.Environmental conditions can be impacted by the temperature, the numberof people in the building, the amount of human traffic in the building,and whether it is day or night. Then, as above, the base stations couldtest the system by attempting to use the tables to determine thelocation of a given base station and then comparing the calculatedlocation to the true location. The table that results in the greatestaccuracy would then be used. In this way, the system could avoid thecomplexity of calculating the signal strength. As above, the systemcould be programmed to environmentally profile itself periodically.

As can be seen from the foregoing detailed description, the presentinvention is directed to a system and a method for determining thelocation of a mobile computer and its user based on the strength ofwireless signals from the WLAN to which the computer is connected. Thepresent invention can also be implemented by a dedicated systemproviding base stations and receiver/transmitters on the mobilecomputer. Similarly, the present invention can operate on any wirelessmechanism, which does not require a line-of-site between the receiverand transmitter, such as radio frequency (RF) transmissions of varyingwavelengths and ultra-sound transmissions. The present invention isequally applicable to a wireless system that interconnects other mobileunits, in addition to mobile personal computer 20, such as cordlessphones, CB radios, two-way radios, or the like.

All of the references cited herein, including patents, patentapplications, and publications, are hereby incorporated by reference intheir entireties.

In view of the many possible embodiments to which the principles of thisinvention may be applied, it should be recognized that the embodimentdescribed herein with respect to the drawing figures is meant to beillustrative only and should not be taken as limiting the scope ofinvention. For example, those of skill in the art will recognize thatthe elements of the illustrated embodiment shown in software may beimplemented in hardware and vice versa or that the illustratedembodiment can be modified in arrangement and detail without departingfrom the spirit of the invention. Therefore, the invention as describedherein contemplates all such embodiments as may come within the scope ofthe following claims and equivalents thereof.

1. A method for determining a location of a mobile unit, comprising:measuring a wireless signal strength; comparing the measured wirelesssignal strength to a table of mathematically estimated wireless signalstrengths and corresponding mobile unit locations; finding a table entrywhose mathematically estimated wireless signal strength is closest, bydistance in signal space, to the measured wireless signal strength; and,determining the location of the mobile unit with reference to the foundtable entry.
 2. The method of claim 1 wherein the determining thelocation of the mobile unit with reference to the found table entryincludes determining the location of the mobile unit to be proximate tothe found table entry's corresponding mobile unit location.
 3. Themethod of claim 1 wherein the finding the table entry whosemathematically estimated wireless signal strength is closest to themeasured wireless signal strength includes finding a plurality of tableentries and wherein the determining the location of the mobile unit withreference to the found table entry includes determining the location ofthe mobile unit to be proximate to a spatial average of the foundplurality of table entries' corresponding mobile unit locations.
 4. Themethod of claim 3 wherein the determining the location of the mobileunit to be proximate to a spatial average of the found plurality oftable entries' corresponding mobile unit locations includes multiplyingeach found table entry's corresponding mobile unit location by aweighting factor prior to the spatial averaging.
 5. The method of claim1 wherein the measuring the wireless signal strength includes measuring,at the mobile unit, a wireless signal strength of a base station, andwherein the table of mathematically estimated wireless signal strengthsand corresponding mobile unit locations includes mathematical estimatesof base station wireless signal strengths at the corresponding mobileunit locations. 6-13. (canceled)
 14. The method of claim 1 wherein themeasuring the wireless signal strength includes measuring, at a basestation, a wireless signal strength of the mobile unit, and wherein thetable of mathematically estimated wireless signal strengths andcorresponding mobile unit locations includes, for mobile units at thecorresponding mobile unit locations, mathematical estimates of mobileunit wireless signal strengths at one or more base stations. 15-22.(canceled)
 23. A computer-readable medium having computer-executableinstructions for performing steps, comprising: measuring a wirelesssignal strength; comparing the measured wireless signal strength to atable of mathematically estimated wireless signal strengths andcorresponding mobile unit locations; finding a table entry whosemathematically estimated wireless signal strength is closest, bydistance in signal space, to the measured wireless signal strength; and,determining the location of the mobile unit with reference to the foundtable entry
 24. The computer-readable medium of claim 23 wherein thedetermining the location of the mobile unit with reference to the foundtable entry includes determining the location of the mobile unit to beproximate to the found table entry's corresponding mobile unit location.25. The computer-readable medium of claim 23 wherein the finding thetable entry whose mathematically estimated wireless signal strength isclosest to the measured wireless signal strength includes finding aplurality of table entries and wherein the determining the location ofthe mobile unit with reference to the found table entry includesdetermining the location of the mobile unit to be proximate to a spatialaverage of the found plurality of table entries' corresponding mobileunit locations.
 26. The computer-readable medium of claim 25 wherein thedetermining the location of the mobile unit to be proximate to a spatialaverage of the found plurality of table entries' corresponding mobileunit locations includes multiplying each found table entry'scorresponding mobile unit location by a weighting factor prior to thespatial averaging.
 27. The computer-readable medium of claim 23 whereinthe measuring the wireless signal strength includes measuring, at themobile unit, a wireless signal strength of a base station, and whereinthe table of mathematically estimated wireless signal strengths andcorresponding mobile unit locations includes mathematical estimates ofbase station wireless signal strengths at the corresponding mobile unitlocations. 28-35. (canceled)
 36. The computer-readable medium of claim23 wherein the measuring the wireless signal strength includesmeasuring, at a base station, a wireless signal strength of the mobileunit, and wherein the table of mathematically estimated wireless signalstrengths and corresponding mobile unit locations includes, for mobileunits at the corresponding mobile unit locations, mathematical estimatesof mobile unit wireless signal strengths at one or more base stations.37-44. (canceled)
 45. A mobile unit comprising: a wireless interfacehardware, wherein the wireless interface hardware obtains a wirelesssignal strength; a memory storage, storing a table of mathematicallyestimated wireless signal strengths and corresponding mobile unitlocations; and a central processing unit, wherein the central processingunit compares the obtained wireless signal strength to the table ofmathematically estimated wireless signal strengths and correspondingmobile unit locations, finds a table entry whose mathematicallyestimated wireless signal strength is closest, by distance in signalspace, to the obtained wireless signal strength, and determines thelocation of the mobile unit with reference to the found table entry. 46.The mobile unit of claim 45 wherein the determining the location of themobile unit with reference to the found table entry includes determiningthe location of the mobile unit to be proximate to the found tableentry's corresponding mobile unit location.
 47. The mobile unit of claim45 wherein the finding the table entry whose mathematically estimatedwireless signal strength is closest to the obtained wireless signalstrength includes finding a plurality of table entries and wherein thedetermining the location of the mobile unit with reference to the foundtable entry includes determining the location of the mobile unit to beproximate to a spatial average of the found plurality of table entries'corresponding mobile unit locations.
 48. The mobile unit of claim 47wherein the determining the location of the mobile unit to be proximateto a spatial average of the found plurality of table entries'corresponding mobile unit locations includes multiplying each foundtable entry's corresponding mobile unit location by a weighting factorprior to the spatial averaging.
 49. The mobile unit of claim 45 whereinthe obtaining the wireless signal strength includes measuring, at themobile unit, a wireless signal strength of a base station, and whereinthe table of mathematically estimated wireless signal strengths andcorresponding mobile unit locations includes mathematical estimates ofbase station wireless signal strengths at the corresponding mobile unitlocations.
 50. The mobile unit of claim 45 wherein the obtaining thewireless signal strength includes requesting, from a base station, awireless signal strength of the mobile unit as measured at the basestation, and wherein the table of mathematically estimated wirelesssignal strengths and corresponding mobile unit locations includes, formobile units at the corresponding mobile unit locations, mathematicalestimates of mobile unit wireless signal strengths at one or more basestations.